After upgrading to thunderbird 60 I discovered that my non-english dictionaries were silently disabled and I can't install them again because thunderbird complains "dictionary can not be installed because it is not compatible with Thunderbird 60..."
To disable the check you have to start TB's config editor, then type
extensions.strictCompatibility and double click the
True to switch it to
false. Don't forget to re-set it to
True after installing the dictionary.